This invention pertains to active reset forward converters employing synchronous rectifiers.
Carsten, "High Power SMPS Require Intrinsic Reliability," PCI Proceedings, Sep. 14, 1981, pp. 495-501, describes a single-ended forward converter comprising a reset circuit sometimes referred to as an "active clamp." The active clamp comprises a capacitor and switch coupled to a power transformer for resetting the transformer and preventing saturation. A similar circuit is discussed by Carsten in "Design Tricks, Techniques and Tribulations at High Conversion Frequencies", HFPC, April 1987, pp. 139-152, and "Techniques for Transformer Active Reset Circuits at High Frequencies and Power Levels", HFPC, May 1990, pp. 235-246. The Carsten articles are incorporated herein by reference.
The Carsten circuits employ an output filter circuit comprising diodes for receiving an AC voltage from the transformer secondary winding and generating therefrom a DC output voltage. It is known in the art to replace such diodes with MOSFETs, e.g. as described by James Blanc in "Practical Application of MOSFET Synchronous Rectifiers," published at the Intelec '91 conference, incorporated herein by reference.
FIG. 1 illustrates a prior art circuit including a reset circuit combined with synchronous rectifiers. FIG. 1 includes a DC input voltage source 1. A main power switch 2 periodically turns on and off for coupling the DC input voltage across a primary winding 3 of an isolation transformer 4. A reset switch 5 and a capacitor 6 are included in this circuit. When main power switch 2 is off, switch 5 is closed, thereby coupling the series combination of capacitor 6 and input voltage source 1 across winding 3. Capacitor 6 typically stores a DC voltage such that the sum of the DC voltage on capacitor 6 plus the DC input voltage from supply 1 is sufficient to reset transformer 4 when switch 5 is closed. Thus, when switch 2 is closed, a positive DC input voltage is applied across primary winding 3, and when switch 5 is closed, a negative DC voltage (equal to the input voltage plus the voltage across capacitor 6) is applied across winding 3 to reset transformer 4.
Transformer 4 includes a secondary winding 7 coupled to a filter/rectifier circuit 8. Filter/rectifier circuit 8 includes synchronous rectifiers 9 and 10, an inductor 11 and a capacitor 12. Circuit 8 receives an output voltage waveform from secondary winding 7 and generates in response thereto a DC output voltage across output leads 13, 14. Synchronous rectifiers 9, 10 are MOS transistors, including parasitic diodes 9d, 10d coupled across their source and drain.
When switch 2 is closed, a positive voltage is present across winding 3, thereby causing a positive voltage across winding 7, which turns on synchronous rectifier 9 and turns off synchronous rectifier 10. When switch 5 is closed, a negative voltage is present across winding 3, thereby causing a negative voltage across winding 7, which turns off synchronous rectifier 9 and turns on synchronous rectifier 10. The advantage of using synchronous rectifiers 9, 10 instead of diodes is that the voltage drop across rectifiers 9, 10 is less than the voltage drop across a typical diode (0.7 volts), and therefore, efficiency of this circuit is enhanced.
FIGS. 2A and 2B illustrate the gate voltage applied to MOS switches 2 and 5, respectively. As can be seen, these gate voltages are out of phase. FIG. 2C illustrates the voltage vp across winding 3 caused by transistors 2 and 5 turning on and off.
Unfortunately, the gates 9g, 10g of MOS synchronous rectifiers 9, 10 are typically very capacitive. FIGS. 2D and 2E illustrate the voltage applied to gates 9g, 10g of synchronous rectifiers 9, 10 by secondary winding 7. As can be seen, there is a small time period in which the voltages at gates 9g, 10g are both high, thereby causing a small time period during which both rectifiers 9, 10 conduct, which in turn causes large current pulses P1, P2 to flow through rectifiers 9, 10 when rectifiers 9, 10 are both conducting. (The current through rectifier 9 is illustrated in FIG. 2F.) It would be desirable to eliminate these large current pulses.